Welcome to prop-sapt’s documentation!

prop-sapt is a Python package for calculations of first-order interaction-induced properties and density matrices in the spirit of SAPT (Symmetry-Adapted Perturbation Theory). It is built on Psi4, which runs the SCF calculations and the integrals; the SAPT expressions are evaluated with NumPy, with density fitting in the MO basis. The analytic property derivatives are described in J. Chem. Theory Comput. 21, 4562 (2025).

Features

  • Interaction-induced properties (e.g. the dipole moment) as analytic derivatives of the SAPT energy corrections

  • Interaction-induced density matrices and their decomposition into SAPT components

  • The SAPT0 / SAPT(DFT) energy decomposition, with uncoupled dispersion

  • Finite-field wrappers for interaction-induced dipoles, from the SAPT energy components (to check the analytic properties) or from the counterpoise-corrected interaction energy of any Psi4 method, e.g. MP2 or CCSD (a supermolecular reference)

  • Densities, orbitals and electrostatic potentials on grids, fragment populations, and .cube file utilities

  • A command-line runner, prop-sapt run, for production calculations from a configuration file

See Installation to install it, and Known limitations before relying on the S^inf exchange-dispersion terms.

Quick Start

import psi4
from prop_sapt import Dimer, calc_property, calc_sapt_energy

psi4.core.set_output_file("output.dat", False)
psi4.set_memory("2 GB")
psi4.set_options(
    {
        "basis": "aug-cc-pvdz",
        "df_basis_scf": "aug-cc-pvdz-jkfit",
        "df_basis_sapt": "aug-cc-pvdz-ri",
    }
)

# the two monomers are separated by "--"
geometry = """
0 1
O  0.000000  0.000000  0.000000
H  0.758602  0.000000  0.504284
H  0.260455  0.000000 -0.872893
--
0 1
O  3.000000  0.000000  0.000000
H  3.758602  0.000000  0.504284
H  3.260455  0.000000 -0.872893
units angstrom
symmetry c1
no_com
no_reorient
"""

dimer = Dimer(geometry)

# SAPT energy components (pandas Series, in hartree), also saved to sapt_results.csv
sapt_energy = calc_sapt_energy(dimer)

# interaction-induced dipole moment, one row per axis (atomic units), also results.csv
dipole = calc_property(dimer, "dipole")
print(dipole[["x1_pol,r", "x1_exch,r", "x2_ind,r", "x2_disp", "x_induced_S2"]])

The Usage Guide guide covers the density matrices, the runner and the other tools.

Contents:

API Documentation:

Indices and tables